The present invention relates to an electromagnetic radiation shutter, and more particularly to a microelectromechanical system (MEMS) dual electrostatic flexible membrane shutter capable of deflecting electromagnetic radiation.
Advances in thin film technology have enabled the development of sophisticated integrated circuits. This advanced semiconductor technology has also been leveraged to create MEMS (Micro Electro Mechanical System) structures. MEMS structures are typically capable of motion or applying force. Many different varieties of MEMS devices have been created, including microsensors, microgears, micromotors, and other microengineered devices. MEMS devices are being developed for a wide variety of applications because they provide the advantages of low cost, high reliability and extremely small size.
Design freedom afforded to engineers of MEMS devices has led to the development of various techniques and structures for providing the force necessary to cause the desired motion within microstructures. For example, microcantilevers have been used to apply rotational mechanical force to rotate micromachined springs and gears. Electromagnetic fields have been used to drive micromotors. Piezoelectric forces have also been successfully used to controllably move micromachined structures. Controlled thermal expansion of actuators or other MEMS components has been used to create forces for driving microdevices. One such device is found in U.S. Pat. No. 5,475,318 entitled xe2x80x9cMicroprobexe2x80x9d issued Dec. 12, 1995 in the name of inventors Marcus et al., which leverages thermal expansion to move a microdevice. A micro cantilever is constructed from materials having different thermal coefficients of expansion. When heated, the bimorph layers arch differently, causing the micro cantilever to move accordingly. A similar mechanism is used to activate a micromachined thermal switch as described in U.S. Pat. No. 5,463,233 entitled xe2x80x9cMicromachined Thermal Switchxe2x80x9d issued Oct. 31, 1995 in the name of inventor Norling.
Electrostatic forces have also been used to move structures. Traditional electrostatic devices were constructed from laminated films cut from plastic or Mylar materials. A flexible electrode was attached to the film, and another electrode was affixed to a base structure. Electrically energizing the respective electrodes created an electrostatic force attracting the electrodes to each other or repelling them from each other. A representative example of these devices is found in U.S. Pat. No. 4,266,339 entitled xe2x80x9cMethod for Making Rolling Electrode for Electrostatic Devicexe2x80x9d issued May 12, 1981 in the name of inventor Kalt. These devices work well for typical motive applications, but these devices cannot be constructed in dimensions suitable for miniaturized integrated circuits, biomedical applications, or MEMS structures.
MEMS electrostatic devices are used advantageously in various applications because of their extremely small size. Electrostatic forces due to the electric field between electrical charges can generate relatively large forces given the small electrode separations inherent in MEMS devices. Referring to FIG. 1 shown is a MEMS flexible membrane electrostatic device 10 as described in detail in U.S. patent application Ser. No. 09/464,010, entitled xe2x80x9cElectrostatically Controlled Variable Capacitorxe2x80x9d, filed on Dec. 15, 1999, in the name of inventor Goodwin-Johansson and assigned to MCNC, the assignee of the present invention. That application is herein incorporated by reference as if set forth fully herein. The MEMS flexible membrane device comprises in layers a substrate 20, a first insulating layer 30, a substrate electrode 40, a substrate insulator 50 and a flexible membrane 60. The flexible membrane is generally planar and overlies a portion of the substrate and, generally, the entirety of the substrate electrode. The flexible membrane typically comprises multiple layers including at least one electrode layer 62 and at least one biasing/insulating layer 64.
The flexible membrane may be defined as having two portions; referred to as the fixed portion 70, and the distal portion 80. The portions are defined horizontally along the length of the moveable composite. The fixed portion is substantially affixed to the underlying substrate or intermediate layers at the flexible membrane to substrate attachment point. The distal portion is released from the underlying substrate or intermediate layers during fabrication of the MEMS device.
Referring to FIG. 2, shown is an alternate embodiment of a MEMS moveable membrane device 100 having a predefined air gap 110 underlying a medial portion 120 of the moveable composite. The medial portion extends from the fixed portion 70 and is held in position or biased without the application of electrostatic force. The air gap results from the release operation employed during fabrication of the MEMS device. During operation the distal portion is free to move, characteristically curling away from the underlying planar surface in the absence of electrostatic forces. The medial portion maintains a non-increasing separation (i.e. the separation is either constant or decreasing) with respect to the underlying planar surface until the flexible membrane begins to bend toward the substrate. As shown an auxiliary biasing layer 130 overlies the electrode layer and structurally constrains the medial portion. By predefining the shape of the air gap, recently developed MEMS electrostatic devices can operate with lower and less erratic operating voltages. For a more comprehensive discussion of MEMS moveable membrane devices having a predetermined air gap see U.S. patent application Ser. No. 09/320,891, entitled xe2x80x9cMicromachined Electrostatic Actuator with Air Gapxe2x80x9d, filed on May 27, 1999, in the name of inventor Goodwin-Johansson and assigned to MCNC the assignee of the present invention. That application is herein incorporated by reference as if set forth fully herein.
Optical displays have been formed that utilize metallized polymer films as one electrode and a second rigid electrode. In application, when voltage is applied between the two electrodes the metallized polymer electrode deflects and is attracted toward the fixed electrode. In particular, the metallized polymer film is typically a rolled up (fully curled) structure prior to application of the voltage as a means of minimizing the overall size of the electrode. Typical prior art optical displays will employ polymer films ranging from 1-4 micrometers in thickness and metal films ranging from 300 to 1000 angstroms in thickness. The display shutters are typically greater than 2 millimeters on a side such that the shutter rolls up to less than 10 percent of the total area. In addition, these shutters have benefited from the use of optically transparent conductive films, such as indium tin oxide (ITO), fabricated on transparent substrates, such as glass, to form an optically transparent fixed electrode.
The present problem is that no film currently exists that is both conductive and completely transparent to a wide frequency range of RF electromagnetic radiation. For any conductive material and frequency of RF electromagnetic radiation, there can be calculated a skin depth of xcex4=sqrt(2/((xcfx89xcexc"sgr")). A layer of conductive material more than a few skin depths in thickness will severely attenuate and reflect incident electromagnetic radiation. The skin depth for a gold film with 40 GHz radiation is 0.37 micrometers. Thus, it is not presently feasible to construct a shutter that has an RF transparent fixed electrode. What is desired is a structure that can serve as a shutter for electromagnetic radiation and, more specifically, a MEMS electromagnetic radiation shutter. A MEMS structure is highly preferred because it offers ease in fabrication, thus minimal cost, and can be operated with relatively low electrostatic power. Such a device would be capable of being implemented in a single MEMS device or in larger macroscopic systems.
The present invention provides for an electromagnetic radiation shutter driven by electrostatic force that is capable of deflecting electromagnetic radiation while benefiting ease in fabrication, low power consumption and minimal cost per unit. Further, methods for using and making the electromagnetic radiation shutter according to the present invention are provided.
An electromagnetic radiation shutter device driven by electrostatic forces according to the present invention comprises a stationary membrane capable of allowing electromagnetic radiation transmission therethrough, and a first and second flexible membrane comprising an electrode element and at least one biasing element. The first flexible membrane has a fixed portion attached to the underside of the stationary membrane and a distal portion adjacent to the fixed portion and released from the underside of the stationary membrane. The second flexible membrane has a fixed portion attached to the topside of the stationary membrane and a distal portion adjacent to the fixed portion and released from the topside of the stationary membrane.
In operation, a voltage differential is established between the electrode element of the first flexible membrane and the electrode element of the second flexible membrane thereby moving the first and second flexible membranes relative to the stationary membrane. In a closed state (i.e. fully activated), the flexible membranes will be generally parallel to the stationary membrane and will capable of deflecting RF electromagnetic radiation. In an open state (i.e. no voltage applied), the flexible membranes will generally be fully curled based on biasing in the flexible membranes induced during fabrication and/or imparted by material characteristics.
In one embodiment of the electromagnetic radiation shutter it will be supported by a frame structure that serves to support the stationary membrane and allow for the unrestrained movement of the flexible membranes from an open state to a closed state and back to an open state. The frame structure may comprise a substrate or other suitable means of supporting the stationary membrane.
In another embodiment of the invention the first and second flexible membranes will comprise a layered construct of a first biasing/insulation layer, an electrode layer and a second biasing/insulation layer. The biasing/insulation layers are typically chosen from a group of materials that are capable of providing the necessary positional biasing to the membrane and the required electrical insulation to the electrodes. The electrode layer comprises a conductive material, such as gold. In instances where gold is used to form the electrode layers, an adhesion-promoting layer may be used between the gold layer and adjacent layers to insure adequate adhesion. The stationary membrane will typically comprise a non-conductive organic insulator material that is capable of allowing RF transmission.
In another embodiment of the invention the electrode elements of the first and second membranes will have a predetermined plan view shape that minimizes power consumption and adds to shutter efficiency. In one such embodiment the electrode elements will be patterned and disposed having a generally tapered plan view shape, with the widest area nearest the fixed portion of the membrane and the narrowest portion furthest from the fixed portion of the membrane. Such a configuration is intended to allow for the analog control of the closing of the flap where different applied voltages correspond to different amounts of unrolling.
Alternatively, another embodiment of the present invention provides for a method of using the electromagnetic radiation shutter of the present invention. The method comprised providing for an electromagnetic radiation shutter having a stationary membrane capable of allowing electromagnetic radiation transmission therethrough, a first flexible membrane attached to the underside of the stationary membrane and comprising an electrode element and at least one biasing element, and a second flexible membrane attached to the topside of the flexible membrane and comprising an electrode element and at least one biasing element. Electromagnetic radiation is then transmitted through the stationary membrane followed by the application of electrostatic voltage across the electrode elements of the first and second flexible membrane. The application of the electrostatic voltage causes the membranes to move from an open state in which the membranes are generally curled to a closed state in which the membranes are generally parallel with the stationary membrane. Once the membranes are in a closed state, generally parallel with the stationary membrane, the electromagnetic radiation is then properly deflected as the application warrants.
Additionally, the invention is embodied in a method of fabricating the electromagnetic radiation shutter of the present invention. The method comprises providing for a substrate, disposing a first release layer on the substrate and then forming a first flexible membrane on the first release layer. A second release layer is then disposed on the first flexible membrane followed by the formation of a stationary membrane on the second release layer, the first flexible membrane and a frame structure. After formation of the stationary membrane, a third release layer is disposed on the stationary membrane followed by the formation of a second flexible membrane on the third release layer and the stationary membrane. The process is completed by removing the first, second and third release layers to allow for a distal portion of the flexible membranes to be released from the stationary membrane.
As such the electromagnetic radiation shutter device driven by electrostatic force is capable deflecting electromagnetic radiation, specifically RF radiation, with minimal power required to activate the shutter. The electrodes in the flexible membranes arc separated by a small gap that makes it possible to apply minimal electrostatic voltage across the electrodes to cause the membranes to uncurl into a closed shutter state. Additionally, the simplistic design of the shutter makes for a device that can be manufactured efficiently and at a minimal cost.